However, there are some unconfirmed reports of intimidation elsewhere in the Niger Delta.

Reuters news agency reports that there are far fewer international election observers present than during previous polls, partly because the entire election cycle was delayed by a week.

Some election officials are doing national youth service and did not turn up after their colleagues were targeted during the previous violence.

The National Youth Corps workers do their service away from their home state and so are seen as vulnerable when violence breaks out.

It is not yet clear to what extent their absence has affected the polling.

European Union and US diplomats have issued a joint statement sharply criticising those behind violence and warning all of Nigeria's leaders to behave responsibly.

The police commissioner in Maiduguri said no-one had been hurt by Tuesday's bombs. Previous bombings in the city were blamed on the Boko Haram Islamist group which has staged frequent attacks in recent months.

One man told the BBC that he was a passenger in a bus which went over a bomb disguised as rubbish and left in a pothole in the road.

"When we got there and drove over the bit of rubbish the bomb then exploded. One tyre burst underneath us with a loud noise," he told the BBC's Focus on Africa programme.

Another man said there were hardly any queues at polling stations in the city, in contrast to previous elections.

Strong opposition challenge

A total of 24 states out of 36 held their gubernatorial elections. Several delayed federal legislative polls also took place.

Ten governors took office years after winning their legal challenges against the 2007 elections and so new polls will not be held in those areas.

Image caption
Fires are still smouldering in some parts of Nigeria after last week's violence

The elections were postponed until Thursday in Kaduna and Bauchi following clashes last week after President Goodluck Jonathan was declared the winner.

Correspondents say the governing People's Democratic Party faces a strong opposition challenge in several areas and is expected to lose some state governorships.

The electoral marathon began with legislative polls on 9 April.

Mr Jonathan, a southern Christian, was appointed to the presidency last year upon the death of incumbent Umaru Yar'Adua, a northerner whom he had served as vice-president.

Many in the north felt the next president should have been from their region, as Mr Yar'Adua died before he could finish his term.

Former military ruler Muhammadu Buhari won most of the mainly Muslim northern states but nationwide only gained half as many votes as President Jonathan.

Analysts say the violence has more to do with poverty and economic marginalisation in the north than religion.

The north and south also have cultural, ethnic and linguistic differences.

Despite the violence, most observers said the elections had been among the best organised since the return of civilian rule in 1999.

To win at the first round, a candidate not only needs the majority of votes cast, but at least 25% of the vote in two-thirds of Nigeria's 36 states. Goodluck Jonathan, of the PDP, reached that threshold in 31 states; runner-up Muhammadu Buhari of the CPC only did so in 16 states.

Nigeria's 160 million people are divided between numerous ethno-linguistic groups and also along religious lines. Broadly, the Hausa-Fulani people based in the north are mostly Muslims. The Yorubas of the south-west are divided between Muslims and Christians, while the Igbos of the south-east and neighbouring groups are mostly Christian or animist. The Middle Belt is home to hundreds of groups with different beliefs, and around Jos there are frequent clashes between Hausa-speaking Muslims and Christian members of the Berom community.

Despite its vast resources, Nigeria ranks among the most unequal countries in the world, according to the UN. The poverty in the north is in stark contrast to the more developed southern states. While in the oil-rich south-east, the residents of Delta and Akwa Ibom complain that all the wealth they generate flows up the pipeline to Abuja and Lagos.

Southern residents tend to have better access to healthcare, as reflected by the greater uptake of vaccines for polio, tuberculosis, tetanus and diphtheria. Some northern groups have in the past boycotted immunisation programmes, saying they are a Western plot to make Muslim women infertile. This led to a recurrence of polio, but the vaccinations have now resumed.

Female literacy is seen as the key to raising living standards for the next generation. For example, a newborn child is far likelier to survive if its mother is well-educated. In Nigeria we see a stark contrast between the mainly Muslim north and the Christian and animist south. In some northern states less than 5% of women can read and write, whereas in some Igbo areas more than 90% are literate.

Nigeria is Africa's biggest oil producer and among the biggest in the world but most of its people subsist on less than $2 a day. The oil is produced in the south-east and some militant groups there want to keep a greater share of the wealth which comes from under their feet. Attacks by militants on oil installations led to a sharp fall in Nigeria's output during the last decade. But in 2010, a government amnesty led thousands of fighters to lay down their weapons.